1. Field of the Invention
The present invention relates to an image recording carrier which accumulates and records a radiograph when irradiated with radioactive rays carrying an image to be radiographed.
2. Description of the Prior Art
Heretofore, there have been known radioactive-energy-accumulating fluorescent substances which accumulate part of radioactive energy when irradiated with radioactive rays, and which emit photostimulated luminescent light depending on the accumulated radioactive energy when irradiated with a beam of visible light or the like. Recent years, CR (computed radiography) has been in increasingly wide use in the medical field and the like. CR is a technique as follows. A radiograph is accumulated and recorded in a radioactive-energy-accumulating fluorescent substance by irradiating the radioactive-energy-accumulating fluorescent substance with radioactive rays having passed through an object. Subsequently, the radioactive-energy-accumulating fluorescent substance is irradiated with excitation light, and thus photostimulated luminescent light is emitted from the radioactive-energy-accumulating fluorescent substance. By reading the photostimulated luminescent light thus emitted, the radiograph is visualized.
There are two types of image reading apparatuses which are widely used for medical CR: a built-in type and a cassette type. In the case of image reading apparatuses of the built-in type, an IP (imaging plate) and a reading section are altogether housed in a single apparatus body, and thus the IP housed in the single apparatus is irradiated with radioactive rays. The IP is obtained by adhering a radioactive-energy-accumulating fluorescent substance to a surface of a substrate. The reading section irradiates the IP with a laser beam or the like, and thus reads the resultant photostimulated luminescent light. Image reading apparatuses of the built-in type will be hereinafter referred to as “built-in apparatuses.” In the case of image reading apparatuses of the cassette type, an IP is housed in a portable cassette. A radiograph is accumulated on the IP through radiographing. This IP is housed in the cassette. This cassette is attached to an image reading apparatus. The image reading apparatus takes the IP out of the cassette, and thus reads the radiograph. Image reading apparatuses of the cassette type will be hereinafter referred to as “cassette apparatuses.”
Built-in apparatuses make it possible to read a radiograph made on the spot, and thus to check on the radiograph immediately. This makes it possible for a user to find a failure in radiographing immediately, and to take another shot. For this reason, built-in apparatuses are widely used for a group physical checkup, which requires radiographs to be taken of multiple objects securely.
On the other hand, in the case of cassette apparatuses, a cassette can be easily moved closer to a part of an object which is intended to be radiographed during radiographing. In a case where, for example, a patient has his/her bone fractured, a cassette apparatus is capable of radiographing an injured part which is intended to be radiographed without forcing the patient to get into uncomfortable poses. In addition, in a case where, for example, an IP is damaged in a cassette, the damaged IP housed in the cassette can be easily replaced with a spare IP. This brings about an advantage that time and costs needed to resume radiographing can be saved to a large extent.
As described above, since the built-in apparatuses and the cassette apparatuses have different advantages, many hospitals are furnished with both a built-in apparatus and a cassette apparatus in many cases. Usually, they use one of the two types of apparatuses depending on an intended use.
Moreover, in recent years, mammography apparatuses have become in use. The mammography apparatuses radiograph a mamma while flattening the mamma with the mamma placed and pressed between an IP and a transmission plate having a radioactive transmittivity. The mammography apparatuses are known for their effectiveness for early detection of breast cancer. In many cases, whether or not hospitals are furnished with a mammography apparatus is an important criterion for patients to choose their hospitals.
There are also two types of mammography apparatuses: a built-in type and a cassette type. In the case of mammography apparatuses of the built-in type, an IP and a reading section in addition to a pressing mechanism and an irradiation unit are altogether housed in a single mammography apparatus body. The reading section reads a radiograph. The pressing mechanism presses a transmission plate toward the IP. The irradiation unit irradiates the IP with radioactive rays. In the case of mammography apparatuses of the cassette type, only elements such as a pressing mechanism and an irradiation unit are installed in a single mammography apparatus body. A cassette in which an IP is housed is attached to the mammography apparatus, and is irradiated with radioactive rays. When using a mammography apparatus of this cassette type, a mamma is radiographed by placing the mamma between the attached cassette and a transmission plate. The cassette which has been used for the radiographing is attached to an image reading apparatus which is equipped separately from the mammography apparatus. Thus, a radiograph which has been accumulated and recorded on the IP is read by the image reading apparatus. In the case of mammography apparatuses of this cassette type, it is easy to replace a damaged IP with a new one. In addition, the conventional cassette apparatuses which have been used in hospitals can be used as image reading apparatuses for reading a radiograph. For these reasons, the mammography apparatuses of the cassette type are more cost-effective and more easy to be introduced than mammography apparatuses of the built-in type.
In a case where a mamma is going to be radiographed by use of a mammography apparatus of the cassette type, a side surface of a cassette is pressed against the base of the mamma (the base of the mamma will be referred to as a “chest wall”), and the mamma is irradiated with radioactive rays while the mamma being flattened on the top surface of the cassette by use of a transmission plate. At this time, the mammography apparatus is incapable of accumulating or recording the radiograph corresponding to a part of the IP housed in the cassette even though the IP is irradiated with the radioactive rays. The part in question extends from a place corresponding to the side surface of the cassette to a place beyond which a radioactive-energy-accumulating fluorescent substance is adhered to the IP. This part in question will be hereinafter referred to as an “unrecorded part.” Because of an unrecorded part of this kind, mammography apparatuses of the cassette type have a problem that a part of a mamma which is closer to the chest wall is missing from a radiograph of the mamma.
With regard to this problem, Japanese Patent Application Laid-open Publication No. 2003-248093 (hereinafter referred to as “JPA-2003-248093”) has disclosed a technique with which an IP is produced by expanding a recording area for accumulating and recording a radiograph in an IP up to the end of a substrate. More specifically, a layer of a radioactive-energy-accumulating fluorescent substance is formed in a place between 0.0 mm and 0.4 mm from the end of the substrate. The technique disclosed by JPA-2003-248093 makes it possible to reduce the unrecorded part in the area, and to accordingly decrease a missing portion of a radiograph.
It is usual, however, that a cassette is configured of a plastic or the like with a thickness of approximately several millimeters. Even though a recording area of an IP is intended to be expanded by use of the technique disclosed by JPA-2003-248093, in reality, a portion of a mamma in a range of the chest wall to approximately 5 mm therefrom is incapable of being radiographed because of the thickness of a plastic of which the cassette is configured. This means that mammography apparatuses of the cassette type are incapable of radiographing a tumor closer to the chest wall until the tumor develops beyond a range of the chest wall to approximately 5 mm therefrom. This incapability places an obstruction to breast cancer checks which aim at earlier detection of breast cancers. On the other hand, if an unrecorded part is intended to be reduced in area by forming a cassette of a thinner plastic, this decreases the strength of the cassette. This brings about a problem that a hard but fragile IP can not be protected by the cassette securely.